I am looking for some advice on job searching while employed. I work in a practice of 3 people but my route of progression would lead to me not getting to do the jobs I enjoy. I have therefore decided to look for a job I feel passionate about. I have been offered an interview for a position I feel I would really enjoy. My question is how do I go about telling my boss if I am offered the new position. I also have had some time off due to a viral infection so feel guilty that I was off sick now I'm job hunting. Hope this makes sense!

Be sure you get the new job in writing before you say anything to your current boss.

This is the way of the business world -- employees come and go, usually for better opportunities. Bosses understand this. I would tell him as soon as you have the new job bolted down. How? Just tell him as you have said it here (but no apologies or feeling guilty).

It is normal to give 2 weeks notice when quitting and going to another job. Your new employer would know this and probably appreciate your honesty.

Thanks for the replies. I have been looking to move on for a few months but with it being such a small practice I feel like I'm letting them down. I guess it's just me being stupid!

Yep, just being stupid, if you can not progess, moving is the normal and regular way that happens.

You find the other job, get it in writing, and secure, then give a two week notice and leave happy.